The present invention relates to end-user cellular communications equipment, and, more particularly, to cellular modems.
The wide spread use of portable cellular telephones in circuit-switched analog wireless data solutions have brought into focus an additional problem--battery life for these portable cellular telephones. For example, a typical mobile-end user configuration is a laptop personal computer (PC) configured with a PCMCIA analog modem that has a "direct connect" interface to the portable cellular telephone. After establishing an circuit-switched analog cellular data connection to a far-end analog modem endpoint, each analog modem expects to receive a constant data carrier signal to maintain synchronization. While it is known to bridge small outages in a data carrier signal caused by cellular impairments like fading, multi-path, etc. (e.g., see U.S. Pat. No. 4,697,281 issued Sep. 29, 1987 to Sullivan), any loss of the data carrier signal for an extended period of time causes the receiving analog modem to drop the data connection. As such, even during periods of mobile-end user inactivity over the data connection, the portable cellular telephone is still powered up (and draining the battery) to continually transmit a data carrier signal from the PCMCIA analog modem to the far-end data communications equipment.
A similar battery problem in circuit-switched analog cellular voice communications is ameliorated by the use of "voice activation technology" (VOX). In particular, it is known that during a voice-only cellular call, if there is a predefined period of silence, then the transceiver of the portable cellular telephone sends a predefined signal (e.g., a tone) to the cellular network and powers-down, e.g., turns off the cellular transmitter to reduce power consumption. It should be noted that the portable cellular telephone does not completely power-off but switches to a lower power state. The predefined signal indicates to the cellular network that the assigned cellular channel should be kept up notwithstanding the loss (temporarily) of the cellular carrier as a result of the power-down. Upon subsequent detection of a voice signal, the transceiver of the portable cellular telephone powers-up to resume communications. This is also referred to in the art as a "Discontinuous Transmission" feature (e.g., see proposed industry standard IS-136.2 "Recommended Text for IS-136 Cellular System Dual Mode Mobile Station-Rasmussen Base Station Traffic Channels and Analog Control Channel Definition of Discontinuous Transmission for Cellular Voice").
Similarly, in the area of packet-switched cellular data communications, known as CDPD (Cellular Digital Packet Data), the discontinuous nature of packet data also allows incorporation of a discontinuous transmission feature to reduce battery drain. Here, a CDPD-compatible portable mobile station autonomously controls the radio transmitter depending on whether there is data to transmit (e.g., see the "CDPD System Specification" from the industry-supported CDPD Forum).
However, in the context of circuit-switched analog cellular data communications the above-described solutions are ineffective. With respect to using the VOX approach, the continuous transmission of a data carrier signal from the cellular modem looks--to the transceiver of the portable mobile phone--like the cellular channel is still in use. Hence, the transceiver will never power-down. Indeed, even if the transceiver powered down, the modems would disconnect due to the disappearance of the data carrier signal. In contrast, the discontinuous transmission feature of the CDPD approach utilizes the inherent discontinuous nature of packet transmission, which is not available in a circuit-switched analog cellular data environment.